Most likely the cable is faulty, or you have the baud rate mis-matched between the PanelDue and the Duet. Both default to 57600 but both can be changed. So:

Check the baud rates are both 57600 (Setup page on PanelDue, and M575 command in config.g)

Check that the wire colours are in the same order (but reversed) at the 4-pin connectors at either end of the cable

Connect a PC running Pronterface or YAT via USB and send M111 S1 P3. This will echo all GCode commands received. If you see lines starting with "aux:" then the wire from the PanelDue Dout pin to the Duet is OK, and the fault probably lies in the wire to Din. Otherwise the Dout wire is faulty.